(1) Field of the Invention
The present invention relates to a process for the production of a dispersion of a water-soluble polymer, which is widely used as, e.g., flocculating agents for waste water treatment, dehydrating agents, chemicals for paper making used in a paper making process, dispersion stabilizers for various suspensions, soil improvers etc.
(2) Description of the Prior Art
Heretofore, as processes for the production of water-soluble polymers used in such a field of the invention as those described above, there are stationary polymerization in an aqueous solution, water-in-oil type emulsion polymerization (Japanese Patent application Laid-Open No. 102388/1979), suspension polymerization in a hydrophobic solvent (Japanese Patent application Laid-Open No. 69196/1979), etc.
Furthermore, for precipitation polymerization in a salt aqueous solution, Japanese Patent application Laid-Open No. 70489/1975 has been disclosed, and for polymerization in a salt aqueous solution mainly comprising acrylic acid at a low pH, Japanese Patent Publication No. 14907/1971 has been disclosed.
In addition, in the U.S. Pat. No. 4,380,600, there is disclosed a process for preparing a water-soluble polymer dispersion by precipitation polymerization in the presence of a water-soluble polymer as a precipitating medium.
In the stationary polymerization in an aqueous solution, since the polymerization is conducted at a monomer concentration of 10% by weight or higher in order to obtain a polymer of a high molecular weight, the product is in a hydrated gel form and hence cannot be easily dissolved as such and therefore it is necessary either to dilute and put it on the market as a flowable, low concentration product or to dry it to a powder. Where it is intended to put on the market a low concentration polymer, it has a drawback that the transport cost is increased, whereas when made into a powder, the heat energy required for drying is great, and also there is a drawback that by heating, a three-dimentional crosslinking is brought about and a water-insoluble part is generated.
The water-in-oil type emulsion is flammable and further has the drawback that valuable organic solvents are wastefully consumed; and with the suspension polymerization in a hydrophobic solvent, since flammable substances such as cyclohexane, toluene etc. are used, there is the drawback that an enormous cost is required for the production apparatus.
Although the precipitation polymerization in a salt aqueous solution is low in the equipment cost and is a good process, it has a drawback that the formed polymer particles tend to stick to each other and grow into a bigger mass and thus handling becomes very difficult.
In the invention described in Japanese Patent Publication No. 14907/1971, the dispersion is stable only under acidic conditions, and when the carboxyl groups are dissociated, it turns to a gel. However, since an anionic flocculating agent generally manifests an effect by dissociating the carboxyl groups and extending the polymer chain, the acidic flocculating agent does not manifest a satisfactory effect if not neutralized upon use except in the case where it is used in an alkaline solution at a high pH. Furthermore, it is difficult for this process to produce a polymer flocculating agent having a degree of anionization of 15 mole % or less which is most effective as a flocculating agent.
In the invention disclosed in U.S. Pat. No. 4,380,600, it is necessary to use a great deal of the expensive depositing polymer so as to insolubilize the desired water-soluble polymer, since the precipitation effect of the depositing polymer is poor.